1. Field of the Invention
The present invention relates generally to a direct access storage device (DASD) of the type in which a read/write transducer head flies above a medium surface when the medium is moving at operating speed, and more particularly to methods for monitoring flying height changes of a transducer head in a disk drive data storage system and apparatus for performing the methods.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. A slider supports one or more magnetic heads. The slider is lightly biased to cause the heads to move toward the recording surface when the disk is stationary; but as the disk is brought up to operating speed, an air bearing is generated which moves each slider and hence the heads away from the recording surface toward a preselected flying height. Achievement of a higher data density on magnetic disks has imposed a requirement to read and write more data on narrower tracks located on the disk surfaces and with increasingly narrow transducing gaps.
Flying height of the transducer head is critical for mechanical and magnetic performance. Transducer heads flying too high perform poorly magnetically, for example, providing a readback signal with a reduced amplitude, a reduced signal to noise ratio and a reduced resolution for bit detection. Transducer heads flying too low are more likely to pick up contamination from the disk that will lead to smears on the heads and to an increase of the hazard of head crashes.
U.S. Pat. No. 4,841,389 discloses a magnetic transducer crash anticipation and response method and apparatus based upon contemporaneous readback measurement indicative of head overwrite efficiency. A signal is written at a frequency f.sub.l on a predetermined dedicated track, read back, then overwritten at a higher frequency f.sub.h. The ratios of the readback signals is used to initiate a desired control operation when the ratio denotes that the flying height is too low.
U.S. Pat. No. 4,777,544 discloses a method and apparatus for insitu measurement of head/recording medium clearance or flying height. The method and apparatus produce relative motion between the magnetic transducer and a magnetic recording medium at a first velocity so that the resulting air bearing positions the magnetic transducer slider at a first flying height from the magnetic medium. A single signal of constant periodicity is written over a predetermined area of the recording medium by the magnetic transducer, and a readback signal is sensed from the predetermined area of the recording medium to produce a first signal. The flying height of the magnetic transducer slider is lowered to substantially zero, and a readback signal is sensed at the lowered flying height to produce a second signal. The first flying height is then calculated as the ratio, expressed in decibels, of the first and second signals times the wavelength divided by a constant. In an alternate embodiment, a plurality of signals are recorded and readback signals are simultaneously sensed at two separate wavelengths. In a further embodiment, a signal is recorded which has a spectral content comprising a plurality of different frequencies, and readback signals are simultaneously sensed at two separate wavelengths.
Disadvantages of many of the above-described and other known arrangements include both the additional hardware and time required for performing the flying height monitoring methods. Further, it is desirable to accurately measure both increases and decreases in flying height.